2022.05.26 Research

Voltage-gated sodium (Nav) channels produce the upstroke of action potentials in excitable tissues throughout the body. The gating of these channels is determined by the asynchronous movements of four voltage-sensing domains (VSDs). Past studies on the skeletal muscle Nav1.4 channel have indicated that VSD-I, -II, and -III are sufficient for pore opening, whereas VSD-IV movement is sufficient for channel inactivation. Here, we studied the cardiac sodium channel, Nav1.5, using charge-neutralizing mutations and voltage-clamp fluorometry. Our results reveal that both VSD-III and -IV are necessary for Nav1.5 inactivation, and that steady-state inactivation can be modulated by all VSDs. We also demonstrate that channel activation is partially determined by VSD-IV movement. Kinetic modeling suggests that these observations can be explained from the cardiac channel's propensity to enter closed-state inactivation (CSI), which is significantly higher than that of other Nav channels. We show that skeletal muscle Nav1.4, cardiac Nav1.5, and neuronal Nav1.6 all have different propensities for CSI and postulate that these differences produce isoform-dependent roles for the four VSDs.

 

A new gating scheme in which both DIII and DIV are necessary for inactivation.
(A) A kinetic model, referred to here as Gating Scheme II. The state C0000 represents the channel when all four voltage sensors are in their resting positions. The rightward transitions reflect the movement of the DIII voltage sensor, followed by DII, and finally DI. These three movements are sufficient to allow the channel to conduct. The transition from the bottom to middle row reflects DIV movement, while the transition from the middle to top row reflects binding of the inactivation motif. See main text for a full description of the model. (B) The G/V (top) and SSI (bottom) curves of Gating Scheme II when parameterized with values reported in Table S4 (black). See Fig. S4 for a full comparison between model outcomes and Nav1.5e data. In red are the model outcomes when the charges associated with DI movement (γ, γi, γ4, δ, δi, and δ4) were reduced and the intrinsic rates varied to reproduce DI-CN data. (C) DII-CN mutant data were reproduced by targeting parameters α2 and β2. (D) DIII-CN mutant data were reproduced by targeting parameters α3 and β3. (E) DIV-CN mutant data were reproduced by targeting parameters α4, α4O, β4, and β4O. Parameter values for all four CN models are reported in Table S5.

This research was achieved by International Collaboration with McGill University (Montreal, Canada). Collaborative experiments on Voltage-clamp Fluorometry were performed at NIPS, with 3 visiting members from McGill University

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Authors:
Niklas Brake, Adamo S Mancino, Yuhao Yan, Takushi Shimomura, Yoshihiro Kubo, Anmar Khadra, Derek Bowie

Journal: Journal of General Physiology
Issue:  2022 Jul 4; 154(7): e202112921
Date:  Epub 2022 May 25
URL:  https://pubmed.ncbi.nlm.nih.gov/35612552/
doi:   10.1085/jgp.202112921.

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